How bad of an offense is this?

I'm working on a level, my first actually. But if I used tilesets from other peoples levels(custom ones), gave them credit, but didn't ask them, is that ok?

It's always best to ask for permission when using other people's stuff that wasn't explicitly declared to be freely usable by their authors.

It's not that bad of an offense, as I bet most authors wouldn't really mind, but it's not endorsed by our community. If you have trouble reaching tileset authors (for example if some of them have left the Forum long ago), you can ask for help, preferably in the Knytt Stories Level Editing Support section.

It's generally considered taboo if 1) you don't give credit, 2) they left notes saying not to use the tilesets in question, or 3) you modified the tilesets without their permission. However, it's always best to ask, and I would definitely advise against using a tileset you had to take from someone's level.

It's like borrowing a guy's lawnmower, using it for a day, and returning it to his garage without ever telling him.  While you haven't done anything wrong as far as he knows, if he does learn about it he has a good chance of being upset.

And if you tell your lawn "hey, just wanted you to know, this is Steve's lawnmower" it makes it somewhat better but not a lot.

This is an offense if the creator wanted to reserve the tileset for his own levels, like Nifflas did with expansions tilesets.